Weeks after the fatal shooting of an unarmed 18-year-old in Ferguson, Missouri, violent nightly protests gave way to the calm deliberations of a grand jury.

It's a group of 12 private citizens charged with deciding whether criminal charges should be brought against police officer Darren Wilson - the man who fired the deadly shots.

How a grand jury works is a mystery to most Americans -- and to much of the world.

The concept of grand juries date from 12th century England, where they were established to protect commoners from overzealous prosecution by the king.

Fifth Amendment

In the United States, that right is enshrined in the Fifth Amendment of the Constitution, which provides that “No person shall be held to answer for a capital, or otherwise infamous crime, unless on a presentment or indictment of a grand jury.”

Authority over grand juries rests with the 50 individual states, and the rules vary from state to state, with activities ranging from indicting serious crimes to investigating criminal activity and the conduct of public officials.

Unlike trial juries, which meet in open court and decide whether a person charged with a crime is guilty or innocent, grand juries only hear evidence presented by a prosecuting attorney.

Charges are brought if the jury determines there is "probable cause" that a crime was committed and that a specific person or persons may have committed it.

A defense attorney cannot strike members from a grand jury, as can be done with a formal trial.

During grand jury proceedings, neither the judge nor the person suspected of committing the crime is present. Proceedings are held in secret to protect reputations of the innocent.

In this case, however, Wilson did testify without a lawyer.

Unique case

In another unusual aspect of this case, the prosecution did not tell the grand jury what crimes Wilson should possibly be charged with, instead leaving it up to the jury to decide what, if any, crimes were committed, according to The Washington Post.

In most cases, after the prosecutor has presented all of the state's evidence, he asks the jury to issue an indictment and leaves the room.

Grand jurors then vote by a show of hands on each of the charges under consideration. In the Ferguson cases, nine "yes" votes are required from the 12-person jury to indict officer Wilson.

According to the Poynter Institute, some key differences between grand juries and juries are that grand juries are not sequestered, they do not have to swear to be impartial and they may do investigating on their own.

Hearsay evidence can be allowed and the bar of proof is much lower - meaning grand jurors only have to find probable cause to issue an indictment.

In a courtroom trial the jury has to be convinced “beyond a reasonable doubt” that a person is guilty of a crime.